Use 1M PHY for the ACK packet from buds to phone direction.

Re-land the setting with feature flag
It could be tested by the below command.
adb shell setprop persist.device_config.aconfig_flags.bluetooth.com.android.bluetooth.flags.asymmetric_phy_for_unidirectional_cis true

Bug: 310187752
Test: manual, create CIS and check the PHY that selected
Test: atest BluetoothInstrumentationTests
Change-Id: I077ce2500a0c97a64701a5acfbe53c9e08f75870

flags/leaudio.aconfig

@@ -48,3 +48,10 @@ flag {
     description: "Support Dynamic Spatial Audio for LE Audio"
     bug: "309665975"
 }
+
+flag {
+    name: "asymmetric_phy_for_unidirectional_cis"
+    namespace: "bluetooth"
+    description: "Apply asymetric PHY setting for unidirectional CIS"
+    bug: "314176433"
+}

system/bta/le_audio/state_machine.cc

@@ -1371,6 +1371,15 @@ class LeAudioGroupStateMachineImpl : public LeAudioGroupStateMachine {
       return false;
     }
 
+    // Use 1M Phy for the ACK packet from remote device to phone for better
+    // sensitivity
+    if (IS_FLAG_ENABLED(asymmetric_phy_for_unidirectional_cis) &&
+        max_sdu_size_stom == 0 &&
+        (phy_stom & bluetooth::hci::kIsoCigPhy1M) != 0) {
+      LOG_INFO("Use asymmetric PHY for unidirectional CIS");
+      phy_stom = bluetooth::hci::kIsoCigPhy1M;
+    }
+
     uint8_t rtn_mtos = 0;
     uint8_t rtn_stom = 0;