Don't clear all of global stats when clearing layer stats.

Otherwise this produces significant server-side skew when validating
stats because pulling layer stats can destroy any pending global stats,
which can cause the loss of an entire day's worth of data.

Bug: 177999557
Test: libsurfaceflinger_unittest
Test: adb shell cmd stats pull-source 10062
Test: adb shell cmd stats pull-source 10063
Change-Id: I60599a584a189a60e7eac9eaba0800d6ade76b90