Fix Integer Overflow in WebmWriter

When size is calculated in estimatedCueSize, the max file size is first
multiplied by 6 and then divided by 1000. The multiplication operation
could result in an Integer Overflow. This change reorders the
multiplication and division to avoid the overflow.

Test: atest StagefrightRecorderTest
Bug: 360998270
Change-Id: Ia2e65cf2ed3cd02aa1f9714c4eef99fbb9b2394e