Improve flakiness of PlatformCompatGating test

The PlatformCompatGating test did not close the ParcelFileDescriptor
opened by UIAutomation.executeShellCommand, which is the likely culprit
of the test throwing a java.io.InterruptedIOException randomly.
The documentation for executeShellCommand requires the users to do the
cleanup themselves.

Test: atest PlatformCompatGating
Bug: 178108878
Change-Id: Iaf4d82d5fe4170931d2c59f2978e545b15e940b0

tests/PlatformCompatGating/src/com/android/tests/gating/PlatformCompatCommandNotInstalledTest.kt

@@ -131,6 +131,10 @@ class PlatformCompatCommandNotInstalledTest {
         assertThat(platformCompat.isChangeEnabled(TEST_CHANGE_ID, appInfo)).isEqualTo(params.result)
     }
 
-    private fun command(command: String) =
-            FileReader(uiAutomation.executeShellCommand(command).fileDescriptor).readText()
+    private fun command(command: String): String {
+        val fileDescriptor = uiAutomation.executeShellCommand(command)
+        return String(ParcelFileDescriptor.AutoCloseInputStream(fileDescriptor).use {
+            inputStream -> inputStream.readBytes()
+        })
+    }
 }