libbinder_ndk: APEX client is considered as platform

When an AIDL interface is built for APEX, its minSdkVersion could be as
low as 29 because there are some APEXes which was launched with Q.
Previously, this didn't cause a problem because __ANDROID_API__ didn't
track minSdkVersion, but sdk_version which actually is
compileSdkVersion. As a result, __ANDROID_API__ was still set to 10000
which means "current", and SharedRefBase::new wasn't marked as
deprecated.

However, as we fix the build system so that __ANDROID_API__ correctly
tracks minSdkVersion, AIDL interface code fails to compile when it is
built for a Q-launching APEX (e.g. DnsResolver). Fix the issue by not
deprecating the new operator also when __ANDROID_APEX__ is defined.
__ANDROID_APEX__ means that the code is built for an APEX which is still
considered as part of the platform, not for a 3p app.

Bug: 163288375
Test: m

Change-Id: Ic923a2da7110e3a60dae3860a24d3954ca355742