mediapc: Lower minimum RAM checked to 5 * 1024 MB

Instead of checking for 6 * 1024 MB as minimum RAM, check for
5 * 1024 MB as getMemoryInfo() returns around 5.4 GB on a device
with 6 GB RAM.

This is derived from commit 0529fa41984b437a75ced77b81447a5f46053447
Bug: 205189942
Bug: 213014017

Test: atest CtsMediaPerformanceClassTestCases:PerformanceClassTest

Change-Id: I5431fd59478d4b514705a3bca9079d97d947e12b
Merged-In: Ib5949879c3c6685304bd715011198d70b428915b